Hard disks for data storage by host microcomputers often contain defects in the media of the disk platters which preclude writing of data at certain locations. During formatting of sectors in the disk tracks, such defects are located and mapped. Methods exist in the prior art for marking defects or "bad blocks" in the media to allow the host microcomputer or the microprocessor disk controller to provide alternate storage locations as replacements.
Typically, an identification field is provided in the data format for each sector formatted onto a disk track. Typically, one bit located in the ID field provides a bad block marker. Assertion of the bad block marker informs the microcontroller or host computer that a defect exists in the sector. Consequently, the sector is unusable for data storage. Data which would normally be stored in the sector must be relocated to another sector on the disk. Some prior art systems provide automatic diversion to spare sectors located on the same track for storage replacement. Certain approaches provide renumbering of the sectors to avoid the bad block.
Other prior art systems provide pointer systems directing the microcontroller or host computer to the alternate sector. One such system replaces the ID field in the bad block with the ID address of the replacement sector.
Defect handling techniques which may be used with minimum impact from a data transfer time standpoint are required. Defect handling in "zero latency" data read systems must accommodate continued data transfer when a bad block is encountered.